This invention relates generally to communication networks and, more particularly, relates to a video encoder for generating information representative of a video frame, a method for transmitting the generated information over a communication channel, and a method for reconstructing the video image from the generated information.
Video communications over a communication network are currently being carried out using source encoders and channel encoders that were designed independently of each other. The tradition of separating the source and channel encoding tasks arises from the theoretical foundation of Shannon""s separation principle of source and channel encoding that espouses the notion that this separation is optimal. According to this principle, the task of the source encoder is to do the best job it can in compressing the input signal so that the compressed bit rate will match the available bit rate of the communication channel.
Shannon""s separation principal does, however, fail to address the error characteristics of the communication channel on which the video information will traverse. Importantly, developing an encoder without regard to these error characteristics has lead to the result that the image signal is rendered effectively useless if even a few bits are corrupted during transmission. This problem becomes especially pronounced in a wireless communication network where large variations and unpredictability in the error characteristics abound. Unfortunately, currently utilized video coding standards including H.261, MPEG-1, MPEG-2 and H.263, which were developed without regard to the error characteristics of the communication channel, fail to provide the error resiliency needed to minimize this problem.
Furthermore, currently utilized error protection and error recovery schemes that are employed to alleviate the noted error-induction problem have the undesirable side effect of wasting bandwidth within the communication channel. For example, a commensurate amount of bandwidth must be allocated to video transmissions at the expense of other transmissions when additional error checking bits are added to the video bit-stream. Furthermore, using an automatic repeat request (ARQ) procedure that functions to initiate the re-transmission of those transmissions deemed to be in error, while providing error recovery, adds both delay and bandwidth overhead to the transmissions. Delay for the case of interactive real-time video applications causes unacceptable application performance, while bandwidth overhead reduces the efficiency of the overall system.
From the foregoing, it is evident that a need exists for software and hardware mechanisms that ensure timely and robust delivery of video data over bandwidth-limited, error-prone communication channels.
In accordance with this need, the subject invention may be generally described as a combined application and network level solution for achieving robust video transmission over bandwidth-limited and error-prone communication channels. More specifically, the desired video transmission is achieved through the use of a method for generating information indicative of a video frame, a method for transmitting the generated information over the communication channel, and a method for reconstructing the video information from the generated information.
For generating the information indicative of a video frame, the video frame is segmented into a plurality of discrete spatial components and each of the plurality of discrete spatial components is transformed into a plurality of discrete frequency components. For transmitting the generated information over the communication channel, the video components are evaluated to determine the peak number of bits required for their transmission, bandwidth is requested as a function of this determination, the most important video components are first transmitted in the requested bandwidth and any remaining bandwidth is used to transmit any remaining video components. For reconstructing the video information from the generated information the video components received are checked for transmission errors and, if any such errors are present, video components from a previous transmission are substituted therefor. In this manner, the invention overcomes the noted deficiencies in the prior art by, among other things, reducing the reliance on error correction and error recovery schemes that are traditionally used in transmission over error-prone channels and preventing bandwidth wastage though intelligent bandwidth reservation and utilization while guaranteeing the promised spatial and temporal video resolution at the receiver.
A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment which is indicative of the various ways in which the principles of the invention may be employed.